In tyres for vehicle wheels, the reinforced structural elements, comprising reinforcing elements, in particular cords and elastomeric matrix that encompasses said reinforcing elements, perform different functions that can be structural, containment or protection functions.
One of the characteristics to be controlled to ensure the integrity of the reinforced structural elements is that the reinforcing elements or cords firmly adhere to the elastomeric material so as to prevent lacerations and breaking down of the compound.
Typically, materials suitable for the reinforcing cords of the layers of the carcass structures, belt structures, and/or bead protection layers (chafers) or reinforcement layers (flippers), can be textile or metal materials, depending on the positioning and/or on the types of tyres and/or of use.
The non-metal materials most commonly used as components of cords for tyres can be natural polymer fibres such as for example rayon and lyocell, or synthetic fibres such as for example aliphatic polyamides (nylon), polyesters and aromatic polyamides (generically known as aramids), selected on the basis of the component within which they are to be incorporated and of the type (for two- or four-wheel vehicles, for heavy-duty vehicles) and depending on the performance requirements such as for example HP (high performance), UHP (ultra high performance), competitions, road or off-road.
The polymeric fibres, typically those made of polyester, and the aramids, present high tensile elastic responses, with elongations that are respectively relatively low or very low, but, by their very nature, in particular the aramids, adhere with difficulty to the tyre.
Adhesive compositions—for example Resorcinol-Formaldehyde-Latex (RFL)—were therefore developed in the tyre sector. These compositions ensure and maintain excellent adhesion of the cords to the tyres, not only in the tyre manufacturing step but above all in the conditions of use thereof and, at the same time, allow materials with very different mechanical properties such as elastomers, which are very elastic, and the above-mentioned fibres, which are rather rigid, to be successfully coupled in a compound. These adhesive RFL-based compositions are applied onto textile cords, generally by dipping. The cords thus treated can be incorporated within an elastomeric matrix to then be assembled together with other semi-finished products in the assembly of a green tyre, which is then shaped and subjected to moulding and vulcanization.
Some fibres, such as rayon and aliphatic polyamides, already acquire optimal properties for coupling with the elastomeric material by means of a single dip in the RFL-based bath (one-step process) while others, such as polyesters or even more so aromatic polyamides, bind with difficulty to the tyres and therefore require special activating pre-treatments of a physical or chemical nature, such as for example the surface activation of the fibres with micro-fibrillations, treatment with ionizing rays, with plasma, with solvent or pre-treatment with a first activating bath (two-step process).
Thanks to the surface pre-activation of the fibres, this pre-treatment, by means of epoxides or isocyanates or other suitable activators, greatly improves the reactivity of the system through the formation of bonds between the pre-treated fibre and the RFL system. Surface pre-activation can take place in the spinning phase through the use of suitable activation baths or directly in the dipping step (double bath process).
Typically, to further strengthen the adhesion of the cords to the tyres, it is possible to introduce specific additives to the compounds that are to cover the cords, such as adhesion promoters, generally comprising a methylene donor, such as for example hexamethylenetetramine, and a methylene acceptor, such as for example resorcinol. During the vulcanization step the cross-linking of these additives allows superior adhesion to be achieved that is more or less elevated depending on the type of compound, promoter, fibre and the treatments thereof.
The RFL system seems to represent the most widespread and effective cord adhesive in use in the tyre sector and, to the knowledge of the Applicant, no alternative systems of actual industrial applicability that achieve such high performance appear to exist.
Albeit resolving, in a more than satisfactory manner, the adhesions problems between cords and tyres, RFL-based compositions are not however ideal for an industrial use on account of the more or less marked toxicity of its reactive components, resorcinol and formaldehyde, both for man and for the environment. In particular, since formaldehyde is a known carcinogenic while resorcinol is suspected to be harmful to the endocrine system, it would be desirable to significantly reduce—if not eliminate—the use thereof in each step of the manufacture of tyres.
Furthermore, t aqueous compositions of RFL are not particularly stable and require a predetermined “aging” period, known as maturation, to reach a sufficient stability for the end use in the present applications.
The high solids content in the RFL emulsions causes, during industrial use, a progressive fouling and build-up of deposits in the various parts of the treatment line; if not appropriately eliminated, these deposits can represent a pollution and fouling risk for woven fabrics treated with RFL.
The regulatory authorities responsible for the classification of chemical compounds in terms of safety are analyzing aspects of health and environment of the components commonly used in the field to date, directing research and the market towards more eco-compatible tyres.
Notwithstanding the above problems, we have been as yet unable to replace the RFL system with other systems having equal performance.
In this regard, some alternative substantially resorcinol- and formaldehyde-free adhesive compositions, which were however potentially toxic and/or showed decidedly inferior adhesive performance than the performance of the RFL treatment and/or were endowed with a low stability and therefore were not suitable for industrial use, were studied and described in the past.
For example, the documents WO2005/080481, US2011104415 (Diolen Industrial Fibres) and EP2426253 (Bridgestone Corp.) describe substantially resorcinol- and formaldehyde-free compositions for adhering reinforcing polyester, polyphenylen-disulfide or thermotropic liquid crystalline polymers elements to tyres, wherein the compositions comprise at least a) a rubber latex, at least b) an epoxide and c) a polyfunctionalized amino cross-linking agent, in aqueous dispersion. A number of possible amino cross-linking agents that are theoretically suitable for the purpose are listed in the description, however experimental data have been provided for just one compound, piperazine.
The Applicant has found that albeit the compositions comprising certain aliphatic polyamines generally suggested in WO2005/080481 or US2011104415 cross-link well, they are too unstable for an effective industrial use. Moreover, many amines, in particular piperazine, are in any case potentially toxic.
GB1,271,883 shows aqueous composition useful for improving the adherence of polyester fibers to the tyres comprising polyamines, in particular polyalkylenamine or polyethylenimine, poly-epoxides and elastomeric polymers.
U.S. Pat. No. 4,472,463 (Goodrich Company) describes an aqueous adhesive emulsion of latex and acrylic resin that is substantially resorcinol-and formaldehyde-free and is used as a second bath for the coating of polyester and aramid fibres to be incorporated in rubber products.
One drawback of this system is that in order to confer an acceptable adhesion to the fibres, they must always be pre-treated with an epoxy composition (double bath), with greater processing times and costs than single dip processes. In addition, the system, which does not also comprise a cross-linking agent, could result in the formation of bonds that are less tenacious with the rubber.
U.S. Pat. No. 7,067,189 (Performance Fibres, Honeywell) shows aqueous adhesive compositions, useful for adhering polymeric fibres to rubber, comprising a modified water-soluble maleinized polybutadiene and, possibly, a latex which, albeit less toxic due to the absence of resorcinol and formaldehyde, do not succeed in matching the performance of the RFL system, not even following optimization of the compounds and/or of the pre-treatment of the fibres.